1. Field of the Invention
The present invention relates to an electric double layer capacitor wherein a dry up phenomenon and Equivalent Series of Resistance (which is hereinafter called xe2x80x9cESRxe2x80x9d) are reduced, and a production method thereof.
This application is based on Japanese Patent Application No. Hei 11-250918, the contents of which are incorporated herein by reference.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
An electric double layer capacitor is small and has large capacity for charge, and has mainly been used for backup of memory or similar. The aim recently has been to develop into some technical fields in which large amounts of current are required such as automobiles, electronic equipment, or similar; research and development with regard to properties of the electric double layer capacitor, particularly, reduction of the ESR has been actively carried out.
FIG. 2 is a sectional side elevation showing an example of a conventional electric double layer capacitor. This electric double layer capacitor has non-electron-conductivity and is composed mainly of a separator 11 made of a porous film having ionic permeability, a pair of polarized electrodes 12 face each other onto a separator 11 and are composed of powdered active carbon or active carbon fiber, or a solid material obtained by solidifying active carbon using a binder xe2x80x9cTeflonxe2x80x9d, a phenol resin, or the like wherein an electrolytic solution permeates into the solid material, gaskets 13 for holding the separator 11 and polarized electrodes 12 from their sides, and a pair of collectors 14 arranged on the outer surfaces of polarized electrodes 12, not on the side of the surface contacting the separator 11, and made of a conductive resin applied conductivity by mixing carbon powder or the like.
Recently, in the field of electronics, equipment has been further miniaturized; consequently, a thin type electric double layer capacitor has been necessary.
If the collector 14 is prepared thinly in order to make the electric double layer capacitor thin, gas permeability of the collector 14 increases. Then, a dry up phenomenon in the electrolytic solution occurs, so that the capacity of the electric double layer capacitor is reduced and the ESR increases.
If the electric double layer capacitor is used requiring a large amount of current, in order to run a large amounts of current from the electric double layer capacitor, it is necessary that the ESR be reduced as much as possible and a voltage drop caused by the ESR is reduced.
However, in a conventional electric double layer capacitor, adhesion of an interface between the collector 14 and the polarized electrode 12 is insufficient and contact resistance therebetween is high, therefore, a problem of the increase in the ESR occurs.
In order to solve the problem of adhesion at each interface between the collectors 14 and the polarized electrodes 12 respectively, an electric double layer capacitor having a conductive butyl rubber which has excellent adhesion to the polarized electrodes 12, as a collector, is disclosed in Japanese Unexamined Patent Application, First Publication, No.7-335494 (Kokai). Furthermore, in order to reduce the ESR, JP 7-335494 discloses the electric double layer capacitor having laminated metal sheets, as a collector, made of metal sheet, for example, aluminum, on which a conductive butyl rubber is laminated.
However, the laminated metal sheet used in the above electric double layer capacitor has insufficient adhesion at the interface between the aluminum sheet and the conductive butyl rubber, so that the contact resistance is large. Particularly, since peeling tends to occur at the interface between the aluminum sheet and the conductive butyl sheet by heating when pressing and adhering the collectors to the gaskets, the contact resistance increases further. Furthermore, as shown in FIG. 3, a conductive material 16 such as carbon particles which is around the surface layer of the conductive butyl sheet 15, is easily moved into the inside of the conductive butyl sheet 15 by heating at high temperature when laminating. Subsequently, in the conductive butyl sheet 15, conductivity may reduce and the ESR may increase.
In the laminated metal sheet composed of the aluminum sheet and the conductive butyl sheet, contact resistance at the interface between the aluminum sheet and the conductive butyl rubber is large and conductivity of the conductive butyl rubber tends to reduce by heating at high temperature when laminating the conductive butyl rubber on the aluminum sheet. Therefore, when a laminated metal sheet is used as a collector, it is difficult that the electric double layer capacitor having low ESR is constantly obtained.
Furthermore, in the above electric double layer capacitor, it was expected to reduce the dry up phenomenon of the electrolytic solution using an aluminum sheet. However, insufficient adhesion at the interface between the aluminum sheet and the conductive butyl rubber causes the dry up phenomenon of the electrolytic solution from the interface and an expected effect, reduction of the dry tip phenomenon could not be obtained.
It is an object of the present invention is to provide an electric double layer capacitor wherein a dry up phenomenon of the electrolytic solution and the ESR are reduced and a production method thereof.
In order to achieve the above objects, the present inventors have diligently carried out research with regard to an adhesion of metal foil and a conductive resin and a method for laminating the conductive resin onto the metal foil, and found that the adhesion of the metal foil and the conductive resin containing a conductive material is improved using a copper foil as a metal foil and the conductivity of the conductive resin is not reduced by forming a conductive film coated with the conductive resin on the copper foil, as a result, the present invention is accomplished.
The electric double layer capacitor of the present invention comprises a separator, a pair of polarized electrodes provided on both sides of the separator, gaskets holding the separator and the polarized electrodes from their sides, and a pair of collectors provided in contact with the outer surfaces of the polarized electrodes and the gaskets, wherein the collectors are a lamination comprising a conductive film formed by coating a conductive resin solution containing a conductive material on at least one surface of a copper foil.
According to the above capacitor, the dry up phenomenon of the electrolytic solution and the ESR are reduced.
The surfaces of the conductive film sides of the collectors may contact to the polarized electrodes.
According to the above construction, adhesion of the collector and the polarized electrode is improved, the contact resistance of the interface between the collector and the polarized electrode is reduced, and the ESR of the electric double layer capacitor is reduced.
Moreover, a resin used in the conductive resin solution may be a styrene-ethylene/butylene-styrene copolymer (SEBS) or an ethylene-propylene-dienemonomer elastomer (EPDM).
According to the above construction, the ESR is further reduced.
The conductive material may be made of graphite or carbon.
According to the above construction, excellent chemical stability is obtained and maintained for a long time without increasing the resistance of the conductive film.
Furthermore, a production method for electric double layer capacitor of the present invention to prepare an electric double layer capacitor comprising a separator, a pair of polarized electrodes provided on both sides of the separator, gaskets holding the separator and the polarized electrodes from their sides, and a pair of collectors provided in contact with the outer surfaces of the polarized electrodes and the gaskets, wherein a conductive resin solution containing a conductive material is coated to at least one surface of copper foil and a conductive film is formed to the surface of copper foil in the collectors.
According to the above production method, the dry up phenomenon of the electrolytic solution and the ESR are reduced.